1. Field of the Invention
The present invention relates to a receiver which can receive terrestrial broadcast or cable distributed digital television (TV) signals, and more particularly to the reception of digital TV signals that are modulated with any of the following methods: the VSB (Vestigial Side Band) modulation method, the QAM (Quadrature Amplitude Modulation) modulation method, and the COFDM (Coded Orthogonal Frequency Division Multiplexed) modulation method.
2. Description of the Background Art
As terrestrial TV broadcast signals change from present day analog signals, such as NTSC, to any of the above digital signal modulation forms, it will become easier to broadcast different TV signals on a series of consecutive adjacent channels as well as on the previously unusable "taboo" channels, thereby increasing the utilization rate of the available channels.
Most present day television receivers employ a single super-heterodyne type tuner which is adequate for the presently sparsely utilized terrestrial frequency spectrum. When the spectrum is filled with consecutive channels of TV signals and the "taboo" channels are used, much like that in a cable TV distribution system, some digital TV signal receivers will employ a dual super-heterodyne type tuner in place of the commonly used single super-heterodyne type to reduce interference in the reception of the desired channel. Such dual super-heterodyne type tuners are designed with the output intermediate frequency signal occupying the same frequency as the single super-heterodyne type tuner's output in order to minimize changes to other parts of the receiver.
In some digital TV signal receivers, the intermediate frequency output of the above dual super-heterodyne tuner is further subjected to one more super-heterodyne process of shifting the tuner's output intermediate frequency signal to an even lower frequency band due to the requirements of the type of signal demodulator or signal processor employed by the receiver system subsequent to the tuner.
One known dual super-heterodyne type tuner designed to receive digital TV broadcast signals is described, for example, by Mizukami et al, in U.S. Pat. No. 5,572,264. According to Mizukami et al, a TV signal is frequency converted a total of three times in total before entering a demodulator. While this method achieves reception of the digital TV signal, it is not an optimum system in terms of minimizing signal distortion. Specifically, local oscillators' phase noise which reduces the performance of the demodulator is introduced into the desired TV signal at three points of the signal path, i.e., at each of the frequency conversion processes performed by mixers.
In such receiver systems, the digital TV signal is frequency-converted a total of three times, and two of the three frequency conversions are consecutive down-conversations.